Cyclohexanedimethanols are important intermediates for producing a variety of polyester and poly(ester-amides) for coatings, fibers, molding plastics, packaging materials, and the like. Cyclohexanedimethanols are typically manufactured by the hydrogenation of the corresponding cyclohexanedicarboxylate esters. For example, one of the more commercially important cyclohexanedimethanols, 1,4-cyclohexanedimethanol (abbreviated herein as “CHDM”), typically is prepared by a two-step hydrogenation process involving hydrogenation of dimethyl terephthalate (abbreviated herein as “DMT”), to give dimethyl 1,4-cyclohexanedicarboxylate (abbreviated herein as “DMCD”), followed by hydrogenation of the ester groups (FIG. 1): 
The second step, involving the hydrogenation of the ester groups, often uses copper containing catalysts. Conventional copper containing catalysts used to reduce the ester groups to diols include copper chromite, copper oxide/zinc oxide, copper oxide/iron oxide, and copper oxide/aluminum oxide. In addition, these catalysts often contain oxides of barium, manganese, aluminum, zinc, or magnesium as promoters. Examples of various processes for the hydrogenation of DMCD to CHDM using copper based catalysts are described in U.S. Pat. Nos. 5,395,987; 5,395,986; 5,395,990; 5,395,991; 5,406,004; 5,414,159; 5,387,753; and 6,187,968; and Japan Patent Application No.'s 10-045646; 2000-7595, and 6-192146. The use of barium-promoted copper chromite for the preparation of CHDM from DMT is described in U.S. Pat. No. 3,334,149. A process for CHDM with enhanced cis isomer content using a copper chromite catalyst devoid of barium is described in International Patent Publication No. WO 0 058 248.
Copper containing catalysts, particularly copper chromite catalysts, are difficult and expensive to dispose of in an environmentally satisfactory manner. In particular, the toxicity of chromium has established a well-recognized need in the art for chromium-free hydrogenation catalysts. Efforts to address that need are exemplified in Japanese Patent Application No.'s 2000-159705; 2000-001447; Hei 10-45645; Hei 6-228028; and U.S. Pat. Nos. 4,837,368; 5,185,476; 5,334,779; 6,294,703. Examples of non-chromium hydrogenation catalysts also include shaped, Raney metal, fixed-bed catalysts doped with rhenium as disclosed in U.S. Patent Application Publication No. 2002/0037808 A1 and U.S. Pat. No. 6,284,703.
Typically, the hydrogenation of dialkyl 1,4-cyclohexanedicarboxylates produces the corresponding 1,4-cyclohexanedimethanol as a mixture of cis and trans isomers. For example, depending on the hydrogenation conditions and conversion, the hydrogenation of DMCD to CHDM using copper chromite catalysts may produce molar cis:trans ratios as high as 1:1 or greater. For many applications such as, for example, high melting polyester or polyester-amide compositions, the trans CHDM isomer is preferred. Examples of processes for the preparation of CHDM having high trans isomer content are described in U.S. Pat. Nos. 5,387,752; 5,395,986; 5,395,987; 5,406,004; and 5,414,159. These processes, however, frequently require the use of chromium-containing catalysts or require forcing conditions or additional purification steps to obtain acceptable conversions and/or purity. It would be desirable to produce CHDM containing a high trans content directly in the hydrogenation process without additional isomer purification or separation steps. Accordingly, it is to the provision of the needs described above that the present invention is directed.